The present invention relates generally to a process for spray coating an object with a waterborne coating and, more particularly, to a process for the treatment and recycling of overspray resulting from such spray coating. The present process is applicable in industry in general and, especially, in the automotive industry where car bodies and a wide variety of other larger objects are spray coated.
Efficient coating of automobiles and other larger objects with liquid coatings normally requires spray application. Current spray application techniques generate a significant amount of overspray and, in order to isolate this overspray from the immediate surrounding environment, the spraying operation is normally conducted in a spray booth.
The overspray occurring, for example, during automobile coating with waterborne coatings is normally removed from the spray booth by an exhaust air stream and/or absorbed in water (water curtains or cascades) circulated from a catchment tank. When it is considered that in one automobile spray booth alone approximately 1-1.5 tons of paints are applied daily and approximately 20-30% of this is overspray, the magnitude of the resulting waste problem is apparent.
There exist essentially two options with respect to handling this waste product--disposal and/or recycling. Because of the large volumes generated, and because of the environmental consequences involved, disposal is a burdensome and very expensive option. Recycling, therefore, becomes the desirable alternative.
Waterborne coatings utilized in general industry, and particularly in the automotive field, can include a large number of solid and/or liquid components. These components can generally be broken down into three primary categories with subcategories as set forth below:
(I) a substantial amount of water; PA1 (II) a variety of resins (generally higher molecular weight organic compounds) including one or more of water-insoluble, water-dispersible and/or water-soluble components; and PA1 (III) optionally, a variety of additives including, for example, water-insoluble (e.g. pigments), water-dispersible (e.g., emulsifiers, stabilizers and rheological additives) and/or water-soluble (e.g. lower molecular weight polar organic co-solvents, acids and amines) components. PA1 (A) water, PA1 (B) a resin part including at least one of a water-insoluble, water-dispersible and/or water-soluble component, and PA1 (C) optionally, an additive part including at least one of a water-insoluble, water-dispersible and/or water-soluble component, PA1 (1) collecting at least a portion of the overspray; PA1 (2a) separating the collected overspray into PA1 (2b) electrophoretically separating the dilute phase into PA1 (3) separating the water fraction into PA1 (4a) recovering the concentrated phase of step (2a); PA1 (4b) recovering the concentrated fraction of step (2b); PA1 (5) recovering the concentrated residue stream of step (3); PA1 (6) generating the recyclable waterborne coating stream by mixing the so-recovered concentrated phase of step (2a), concentrated fraction of step (2b) and concentrated residue stream of step (3) in a manner and with other components necessary to result in a mixture having substantially the same composition as the waterborne coating. PA1 (1) collecting at least a portion of the overspray; PA1 (2) electrophoretically separating the collected overspray into PA1 (3) separating the water fraction into PA1 (4) recovering the concentrated fraction of step (2); PA1 (5) recovering the concentrated residue stream of step PA1 (6) generating the recyclable waterborne coating stream by mixing the so-recovered concentrated fraction of step (2) and the concentrated residue stream of step (3) in a manner and with other components necessary to result in a mixture having substantially the same composition as the waterborne coating. PA1 (1a) means for collecting at least a portion of overspray; PA1 (1b) a catchment tank for holding the collected overspray; PA1 (2) electrophoretic separating means, connected to the catchment tank, for electrophoretically separating the collected overspray into PA1 (3) means, connected to the electrophoretic separation means, for separating the water fraction into PA1 (4) means, connected to the electrophoretic separation means, for recovering the concentrated fraction; PA1 (5) means, connected to the means for separating the water fraction, for recovering the concentrated residue stream; PA1 (6) mixing means for generating a recyclable waterborne coating stream from the concentrated fraction, the concentrated residue stream and other components necessary to result in a mixture having substantially the same composition as the waterborne coating, PA1 wherein the recyclable waterborne coating stream comprises substantially the same composition as the waterborne coating, and the recyclable water stream contains substantially none of the resin or additive parts of the waterborne coating. PA1 (1a) means for collecting at least a portion of the overspray; PA1 (1b) a catchment tank for holding the collected overspray; PA1 (2a) means, interposed between the catchment tank and an electrophoretic separation means, for separating the collected overspray into PA1 (2b) electrophoretic separating means, connected to the means for separating the collected overspray, for electrophoretically separating the dilute phase into PA1 (3) means, connected to the electrophoretic separation means, for separating the water fraction into PA1 (4a) means, connected to the means for separating the overspray, for recovering the concentrated phase; PA1 (4b) means, connected to the electrophoretic separation means, for recovering the concentrated fraction; PA1 (5) means, connected to the means for separating the water fraction, for recovering the concentrated residue stream; PA1 (6) mixing means for generating the recyclable waterborne coating stream from the concentrated phase, the concentrated fraction, the concentrated residue stream and other components necessary to result in a mixture having substantially the same composition as the waterborne coating.
The sheer variety of components makes effective treatment and recycling a difficult task.
The currently practiced recycling procedures for waterborne coatings require the addition of coagulating agents and/or detackifiers to the collected overspray. The resulting coagulate is extracted by sedimentation, flotation, filtration and/or similar separation processes, and subsequently removed from the spraying area. Due to the presence of the added chemicals, and due to the coagulated form of the recovered product, recycling is a costly and complex process. In addition, the chemical additives often affect the properties of any reclaimed product, and separation of many water-soluble materials, such as low molecular weight organic cosolvents, acids and amines, remains a problem.
Various other recycling methods have also been proposed, but none have been totally successful. For example, WO82/02543 describes a method for recovery and recycling of waterborne coating overspray, whereby the overspray is collected and separated (e.g. by a traveling screen filter or a settling tank) into a low and high solids (e.g. 10-50% solids) fraction. The solids content of the high solids fraction is preferably adjusted to fall within the range of 5-30% solids (e.g. by the addition of water), then passed in the form of a slurry to a decanter centrifuge, of a type well-known in the art, which further separates the liquid from the solids. The solid component is said to comprise about 75 wt % solids, which can be utilized as a base for a reclaimed paint product, or can be further dried for disposal at a landfill. The liquid component, which can be recycled back to the spray booth, is said to typically comprise less than 1 wt % solids. As is clear, the separation methods described in the reference are generally ineffective for separating water-soluble components and, especially, the aforementioned lower molecular weight water-soluble components which tend to build up in the recycled water.
U.S. Pat. No. 4,607,592 discloses a method of processing overspray in conjunction with the use of a complicated apparatus and a burdensome procedure. Separation is accomplished via a complex and special filter and filtration technique into concentrated and clear water fraction. The concentrated fraction is said to contain the solids and water-soluble components of the overspray, with the composition of the concentrated fraction being automatically adjusted to meet the composition of the fresh paint. The reclaimed paint is then recycled by adding it to the fresh paint feed.
While it appears from U.S. Pat. No. 4,607,592 that effective separation and recycling can be accomplished through complex and burdensome means, it would be highly desirable to develop a relatively simple yet effective and efficient recycling method for such waterborne "waste" products which avoids the use of the coagulating and detackifying chemicals mentioned above, and in addition effectively and efficiently recovers all of the components in a form suitable for direct recycling.